The definition
An explosive atmosphere is a mixture of flammable gas, vapour, mist or dust with air, in a proportion such that, once ignition begins, combustion spreads through the entire mixture. “Through the entire mixture” is the decisive part of the definition: it is the difference between a spark that flares and a volume that explodes.
Three conditions
- Flammable material — at a concentration between the explosive limits.
- Oxygen — normally the oxygen in air is enough.
- An ignition source — with energy sufficient to ignite that mixture.
If the three are not in the same place at the same time, there is no explosion. That is why protection is not a single measure but work carried on three fronts at once.
The order of protection
- Prevent the atmosphere forming. Substitute the flammable material, keep it in a closed system, ventilate, hold the concentration below the explosive limit.
- Prevent ignition. Classify the zones, use equipment suited to the zone, earth away static, put hot work under permit.
- Limit the effect. Explosion venting, suppression, isolation, pressure-resistant construction; keep people away from the effect.
The order is not arbitrary: if the first step succeeds, the other two are never tested.
Two things often confused
A flammable liquid is not itself an explosive atmosphere; its vapour becomes one when mixed with air. Likewise a heap of dust is not an explosive atmosphere; it becomes one when it is lifted into a cloud. That distinction is the source of the most common assessment error in dusty plants: with the floor clean, the risk looks absent — while the real hazard is accumulated dust that one vibration will lift.